Anodic spark reaction processes and articles



Dec. 20, 1966 w. M NEILL ETAL ANODIC SPARK REACTION PROCESSES ANDARTICLES 2 SheetsSheet 2 Filed Sept. 17, 1963 BSVL'IOA INVENTORS WILLIAMMcNElLL BY LEONARD L. GRUSS ATTORN EYSI United States Patent 3,293,158ANODIC SPARK REACTION PROCESSES AND ARTICLES William McNeill,Philadelphia, and Leonard L. Gruss, Willow Grove, Pa., assignors to theUnited States of America as represented by the Secretary of the ArmyFiled Sept. 17, 1963, Ser. No. 309,580 21 Claims. (Cl. 204-56) Theinvention described herein may be manufactured and used by or for theGovernment for governmental purposes without the payment of us of anyroyalty thereon.

This invention relates to anodic spark reaction and particularly, toprocesses for coating metals by anodic spark reaction and to coatedarticles produced by such processes. The invention also relates to aprocess for coating assembled combinations of different metals and toassemblies coated thereby.

The phenomenon of anodic sparking has been observed by manyinvestigators and is described in detail in the prior art. The effectappears when the voltage across an anodic barrier film is raised to apoint where film thickness can no longer increase uniformly, anddielectric breakdown occurs. The term anodic barrier film is used toindicate a continuous film which covers an anode surface and acts as abarrier to the passage of electric current in the anode direction. Mostof the prior art on anodic barrier films, US. Patent 2,364,964 beingrepresentative thereof, is concerned with the phenomena which occur atvoltages below the spark potential, and consequently little is knownabout the chemical and structural properties of the products of anodicreactions that occur above the spark potential.

Application of the principals of anodic spark reaction has been made inthe protective finishing of light metals, particularly magnesium. In theHAE process (US. Patent 2,723,952) an electrolyte containing OH, PO F1AlO and MnO ions is used and magnesium is anodized at voltages above thespark potential. A hard, adherent layer containing a mixture ofrefractory magnesium compounds is thereby deposited on the magnesiumsurface. The same type of reaction is employed in the Cr-22 process (US.Patent 2,778,789) but the electrolyte is an ammoniacal solution of CrOHPO and P ions and the resulting coating is non-alkaline. While theprior art applications of anodic spark reaction suggest the possibleformation of anodic barrier films, there has been little evidence as tothe structure or composition of the anodic reaction products andattempts to utilize anodic spark reaction to form protective or otheruseful coatings on other than the usual metals of Al, Mg and alloysthereof have met with little success. The problem becomes increasinglycomplex where, as in the production of many hardware items, it isnecessary prior to finishing to complete an assembly of components madefrom different metals, e.g., Al and Mg assemblies containing threadedsteel inserts.

Accordingly, a principal object of the present invention is to provideimproved anodic spark reaction processes for providing useful coatingson a variety of metals.

Another object of the invention is to provide an improved anodic sparkreaction process for providing useful coatings on assemblies ofcomponents made from different metals.

Other objects of the invention will in part be obvious and in partappear hereinafter in the following detailed description of theprinciples of the invention and several of the embodiments thereof.

In the drawings accompanying and forming a part of the specificationFIG. 1 depicts voltage-time curves for various metals subjected toanodic spark reaction in aluminate solution,

3,293,158 Patented Dec. 20, 1966 FIG. 2 depicts voltage-time curves forvarious metals subjected to anodic spark reaction in tungstate solution,and

FIG. 3 depicts voltage-time curves for various metals subjected toanodic spark reaction in silicate solution.

In the course of the investigation leading to the present invention, anumber of metals were subjected to anodic spark reaction in variouselectrolytes including aqueous solutions of NaAlO Na WO -ZH O, or Na SiO'9H O. The anodic reactions were carried out at a current density in therange of 0.1 to 1.0 amps/in. with the voltage across the electrolyticcell being increased until sparks appeared on the anode surface, andthereafter increased; manually to maintain the required current as theanode film resistance increased. Electrolyte temperature was maintainedin the range 0 to 40 C., and treatment time required for coverage of theanode varied with current density, the process being more rapid ascurrent density increased. FIGS. 1, 2 and 3 depict the anodic voltagesplotted as a function of time for the various metal parameters in 0.1 Nsolutions of NaAlO Na Wo -2H O, and Na SiO 91-1 0, respectively.

In obtaining the data plotted in FIGS. 1, 2 and 3, a current density of0.16 amp/cm. was employed in all reactions, except with Ni in Na SiO forwhich the current density was 0.32 amp/cmn": The reaction cell was aglass vessel which was thermostatted at 25il C., and was equipped with astirring device and a cathode comprising .an inert conductor, e.g.,platinum or graphite. The cathode surface area was at least equal to theanode surface area in all experiments. The power supply for this seriesof experiments was a rectifier having a variable DC output of 0 to 5000volts. The voltage output was unregulated, but fluctuations were lessthan 6 percent.

Anodes were prepared in the form of cylindrical rods and mounted intight-fitting Teflon sleeves which served to mask the anode surface atthe air-electrolyte interface. Anodes varied slightly in size, buttypical dimensions were 0.8 cm. dia. with a total surface area of about6.5 cm. exposed to the electrolyte. The only exceptions were the goldelectrodes which were 2 cm. in length and 0.1 cm. dia., and the Mnelectrode which was a fiat plate of approximately 4 cm. surface area.Surface preparation also varied, depending on the anode metal, but thegeneral procedure was to degrease in an organic solvent, etch in anappropriate acid or alkaline solution and rinse in distilled water.Cleaning procedures were carried out immediately prior to anodizing.

The analyses of anodic spark reaction products were performed oninsoluble material which adhered to the anode. Powder X-ray diffractionpatterns were obtained (Copper Ka radiation and at 57.3 mm. camera) onproducts of all the anodic spark reactions shown in Table I. Chemicaland spectrographic analyses were made of anode products from aluminatesolutions. spectrographic analyses were performed on the samples toprovide a qualitative check on their elemental composition. Theseanalyses showed that the only metal ions in the anode products werethose of aluminum and the anode metal.

The spark reaction products adhering on anodes treated in NaAlOsolutions were generally light in color, very hard, and had theappearance of porous sintered powders.

Chemical analyses were made of the spark reaction products obtained inNaAlO solutions and the results are summarized in Table II. It isevident from the results that major components of the spark reactionproducts are derived from the anion constituent ofthe electrolyte.

X-ray analyses of the spark reaction products from NaAlO solutions aregiven in Table III. The X-ray data do not account for all the anodeproducts which were shown in the chemical analyses, and the probablereason is that there were significant quantities of noncrystallinematerial present. It was in fact, surprising that the X-ray patterns inthe present study were sufficiently Well-defined to permit unambiguousidentification of the major anode products ll-A1 MgA1 O and ZnAl O andMgO.

Spark reactions were obtained with Al, Cd, Zn, Bi, and Cu in Na WOsolutions, but not with Mg, Ni, Co, Fe, and Ag. The main spark reactionproduct was in all cases a chalky yellow or yellow-green powder thatformed a thick coating on the anode. The coating on the cadmium anodecontained a blue powder which was shown to be of the same composition asthe yellow-green material. X-ray diffraction patterns obtained of theTABLE TABLE IL-CHEMICAL ANALYSES OF ANODIO SPARK REACTION PRODUCTS IN0.1 N NaAlOz 1 The oxidation state of the anode metal ions is assumed tobe plus 2 'lhe MgO in the anode product is assumed to result from the Mgconstituent in the aluminum alloy anode.

IIL-ANODIC SPARK REACTION PRODUCTS Anode Metal Electrolyte 0.1 N NaAlO0.1 N NagWO; 0.1 N Na SiO MgAl O4+MgO N.I. ZnSiO Bi 1 1 Metallic phase.

N.I. X-ray difiraction pattern could not be identified. Notinvestigated.

N.R. no spark reaction.

products from the Na WO solutions could be superimposed on a pattern for99% pure monoclinic W0 which showed that the crystalline anode productwas essentially this material.

Tungsten trioxide, W0 in its polycrystalline form is useful as acatalyst in a number of chemical syntheses,

TABLE L-ANODIC GLOW SPARK INITIATION VOLT- Electrolyte Anode Metal 0.1 NNaAlOz 0.1 N NflzWO-1 0.1 N N a SiO Glow Spark Glow Spark Glow Spark 285N.R NR.

110 N.R N.R 265 180 i Not investigated. N.R., no spark reaction.

and as a high temperature corrosion protective coating for titanium. Itis thus seen that the present invention provides a method of directlyforming polycrystalline coatings of anhydrous W0 on several metals.While it has been shown in the prior art that hydrated W0 can beprecipitated from tungstate solutions by anodic reaction, it isnecessary in such cases to provide an additional dehydration step inorder to obtain anhydrous material.

The anode products from reactions in Na SiO solutions bore someresemblance to the slags which form on furnace refractories. The resultsof X-ray diffraction analyses of these materials are given in Table III.Crystalline phases were found on all the anodes except Cd. A very heavybackground in the films for Bi, Ni, Cu, Zn, and Fe anode productspointed to the presence of amorphous material, e.g., fused silicates.The crystalline phases reported in Table III were well characterizeddespite the high background level and represent major anode productcomponents. The presence of metallic Cu, Fe, and Bi in the coatings onthese anodes did not impart electrical conductivity to them, and it isprobable that the metallic phases were embedded in other anode products,e.g., amorphous silicates.

While silicate coatings for the protection and decoration of metalsurfaces are well known, typical processing steps in the application ofsuch coatings are slip-casting or spraying of powdered silicate slurriesto obtain soft coatings which must then be fired in order to be hardenedand bonded to the underlying metal surface.

The present invention provides a method for producing hard, adherentsilicate coatings on various metals by a one-step electrochemicalprocess, which consists of anodization of metal articles in a relativelydilute aqueous silicate electrolyte. While it is recognized that, ingeneral, silicate solutions are not new in electrochemical processes,heretofore there has been no recognition of the critical importance ofthe nature and concentration of the anion constituent in determining thecomposition and structure of anodic reaction products.

The aforementioned results showed clearly the operability of anodicspark reaction to form useful coatings on a variety of metals when thecritical nature and concentration of the anion constituent in theelectrolyte is understood. These results cannot be attributed to therelatively high current density which was employed (0.1-6 amp/cm?)because in a number of cases, e.g., Mn in NaAlO and Mg, Ni, Co, Fe, andAg, in Na WO solution, no such phenomena could be observed. This wastrue even when the current density was increased to several times theabove value. Furthermore, the growth of a barrier film was found tooccur in a number of cases, e.-g., Ni and Co in NaAlO solution, atcurrent densities less than one tenth the above value.

Confirmation of the almost universality of. the inventive process wasobtained when anodic spark reaction was employed to :form usefulcoatings on refractory metals, e.=g., Ti, Zr, V, Mo, W, Nb and Ta.Visible sparking on anodes of the refractory metals was observed in allcases at a DC. initiation voltage in excess of 190 volts. Anodicreaction products forming coatings on the refractory metal anodescomprised corundum (oi-A1 0 or structurally related analogues (MlgAl QZnA1 O etc), anhydrous W0 and fused silicates, respectively, where theanion constituent of the electrolyte employed consisted of aluminateion, tungstate ion, and silicate ion, respectively. Concentrations ofanion constituents were varied from an effective amount up to about 0.3N 11501 aluminate ion and from an effective amount up to about 0.2 N fortungstate ion and silicate ion.

In accordance with another embodiment of the invention, theaforementioned coatings of corundum and structurally related analoguesthereof may be formed on assembled combinations of the aforedescribedmetals in an electrolyte wherein the anion constituent consists ofaluminate in an effective amount up to about 0.3 N. In one demonstrationa bar of aluminum exposing inserted pins of Cu, Fe, Ni, Mg and Zn wasanodized at a current density of approximately 0.5 amp/in. in a 0.1 Nsodium aluminate solution, the temperature of which ranged from 25 to 40C. during treatment. The anode voltage rose from O to approximately 420volts, DC, in a period of to minutes at the end of which time all themetal surfaces of the assembly were covered with hard coatings ofcorundurn or spinel, said coatings being sufficiently hard to scratchglass.

Criticality of the concentration of the aluminate anion constituent inthe electrolyte employed is shown by results of anodic spark reactionattempts set forth in the following table:

TABLE IV.RESULTS OF ANODIC SPARK REACTION Having thus described theinvention so that others skilled in the art may be able to understandand practice the same, it is expressly understood that the invention isnot limited to the preferred embodiments but may be otherwise embodiedor practiced within the scope of the following claims.

We claim:

1. An electrochemical process for producing a hard, adherent coating onthe surface of an article comprising at least one metal selected fromthe group consisting of Al, CO: a V, M0 W; and Ta, said processcomprising subjecting .said metal to anodic spark reaction in an aqueouselectrolyte wherein the anion constituent consists of alnminate ion in aconcentration ranging from an effective normality up to 0.3 N.

2. An electrochemical process according to claim 1 wherein an anodiccurrent density between about 0.1 and 1.0 amp/in. is maintained duringsaid anodic spark reaction.

3. An electrochemical process according to claim 1 wherein saidelectrolyte is maintained at a temperature between 0 and 40 C. duringsaid anodic spark reaction.

4. An electrochemical process for producing a hard, adherent coatingcomprising a substance selected from the group consisting of corundumand structurally related analogues thereof on the surface of a metalselected from the group consisting of Al, Zn, -Bi, Ni, Co, Fe, Cn, Ag,Au, Ti, Zr, V, Mo, W, Nb, and Ta, said process comprising subjectingsaid metal to anodic spark reaction while maintaining anodic currentdensity between about 0.1 and 1.0 amp/in. in an electrolyte consistingof an aqueous solution of sodium aluminate ranging in concentration froman effective normality up to 0.3 N and maintained at a tempertaurebetween 0 and 40 C.

5. An electrochemical process for producing a hard, adherent coating onthe surface of. an assembly of different metals, said process comprisingsubjecting said assembly to anodic spark reaction in an electrolytewherein the anion constituent consists of aluminate ion in aconcentration ranging from an effective normality up to 0.3 N.

6. An electrochemical process according to claim 5 wherein said coatingcomprises a substance selected from the group consisting of corundum andstructurally related anolo-gues there-of.

7. An electrochemical process according to claim 5 wherein an anodiccurrent density between about 0.1 and 1.0 amp/in. is maintained duringsaid anodic spark reaction.

8. An electrochemical process according to claim 5 wherein saidelectrolyte is maintained at a temperature between 0 and 40 C. duringsaid anodic spark reaction.

9. An electrochemical process for producing a hard, adherent coatingcomprising a substance selected from the group consisting of corundum'and structurally related analogues thereof on the surface of anassembly of different metals selected from the group consisting of Mg,Al, Zn, Bi, Ni, Co, Fe, Cu, Ag, Au, Ti, Zr, V, M-o, W, Nb, and Ta, saidprocess comprising subjecting said assembly to anodic spark reactionwhile maintaining anodic current density between about 0.1 and 1.0amp/in. in an electrolyte consisting .of an aqueous solution of sodiumaluminate ranging in concentration from an effective normality up to 0.3N and maintained at a temperature between 0 and 40 C.

10. The method of coating the surface of a metal selected from the groupconsisting of Al, Cd, Zn, Bi, Cu, Ti, Zr, W, Nb, and Ta, which compriseelectrolytically treating which comprises electrolytic treatment of ananode of said metal in an electrolyte consisting of an aqueous tungstatesolution at a voltage sufliciently high to cause sparking on the anodesurface, the concentration of tu-ngstate anion in said solution rangingfrom an effective normality up to 0.2 N.

11. The method of claim 10 wherein the electrolyte temperature duringtreatment is maintained between 0 and 40 C.

12. The method of. claim 10 wherein the anodic current density ismaintained between 0.1 and 1.0 amp/in 13. The method of producing ananhydrous tungsten brioxide coating on the surface of a metal selectedfrom the group consisting of A1, Cd, Zn, Bi, Cu, Ti, Zr, V, Mo, W, Nb,and Ta, which comprise selectrolytically treating said metal at avoltage sufliciently high to cause sparking on said surface and at acurrent density between 0.1 and 1.0 amp/in. in an aqueous tungstatesolution maintained at a temperature between and 40 C., theconcentration of tungstate anion in said solution ranging from aneffective normality up to 0.2 N.

14. The method of producing a silicate coating on the surface of a metalselected from the group consisting of Al, Zn, Bi, Ni, Fe, Cu, Ti, Zr, V,Nb, Ta, M0, and W, which consists of the electrolytic treatment of ananode of said metal in an electrolyte consisting of an aqueous silicatesolution at a voltage sufiiciently high to cause sparking on the anodesurface, the concentration of siltcate anion in said solution rangingfrom an etfective normality up to 0.2 N.

15. The method of claim 14 in which the electrolyte temperature is inthe range 0 to 40 C.

16. The method of claim 14 in which the anodic current density is in therange 0.01 to 1.0 amp/i11 17. The method of producing a silicate coatingon the surface of a metal selected from the group consisting of Al, Zn,Bi, Ni, Fe, Co, Ti, Zr, V, Nb, Ta, Mo, and W, which compriseselectrolytically treating said metal at a voltage s-ufficiently high tocause sparking on said surface and at a current density between 0.1 and1.0 amp/in. in an aqueous silicate solution maintained at a temperaturebetween 0 and C., the concentration of silicate anion in said solutionranging from an efiective normality up to 0.2 N.

18. A coated article produced by the process of. claim 1.

19. A coated article comprising an ass. 'bly of different metalsproduced by the process of claim 5.

20. A coated article produced *by the process of claim 10.

21. A coated article produced by the process of claim 14.

References Cited by the Examiner UNITED STATES PATENTS 1,954,000 4/ 1937Truesdale et al. 2045 6 2,196,161 4/1940 Frasch 204-5-6 2,215,167 9/1940Sumner et a1. 20456 X 2,313,755 3/1943 Loose 20456 2,348,826 5/1944Kra-use et a1. 20456 2,364,964 12/ 1944 Frasch 204-58 2,780,591 2/1957Frey 2 0456 X FOREIGN PATENTS 543,726 3/ 1942 Great Britain.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

1. AN ELECTROCHEMICAL PROCESS FOR PRODUCING A HARD, ADHERENT COATING ONTHE SURFACE OF AN ARTICLE COMPRISING AT LEAST ONE METAL SELECTED FROMTHE GROUP CONISTING OF AL, ZN, BI, NI, CO, FE, CU, AG, AU, TI, ZR, V,MO, W, NB, AND TA, SAID PROCESS COMPRISING SUBJECTING SAID METAL TOANODIC SPARK REACTION IN AN AQUEOUS ELECTROLYTE WHEREIN THE ANIONCONSTITUENT CONSISTS OF ALUMINATE ION IN A CONCENTRATION RAGING FROM ANEFFECTIVE NORMALITY UP TO 0.3 N.
 10. THE METHOD OF COATING THE SURFACEOF A METAL SELECTED FROM THE GROUP CONSISTING OF AL, CD, ZN, BI, CU, TI,ZR, W, NB, AND TA, WHICH COMPRISES ELECTROLYTICALLY TREATING WHICHCOMPRISES ELECTRLYTIC TREATMENT OF AN ANODE OF SAID METAL IN ANELECTRLYTE CONSISTING OF AN AQUEOUS TUNGSTATE SOLUTION AT A VOLTAGESUFFICIENTLY HIGH TO CAUSE SPARKING ON THE ANODE SURFACE, THECONCENTRATION OF TUNGSTATE ANION IN SAID SOLUTION RANGING FROM ANEFFECTIVE NORMALITY UP TO 0.2 N.
 14. THE METHOD OF PRODUCING A SILICATECOATING ON THE SURFACE OF A METAL SELECTED FROM THE GROUP CONSISTING OFAL, ZN, BI, NI, FE, CU, TI, ZR, V, NB, TA, MO, AND W, WHICH CONSISTS OFTHE ELECTROLYTIC TREATMENT OF AN ANODE OF SAID METAL IN AN ELECTRLYTECONSISTING OF AN AQUEOUS SILICATE SOLTION AT A VOLTAGE SUFFICIENTLY HIGHTO CAUSE SPARKING ON THE ANODE SURFACE, THE CONCENTRATION OF SILICATEANION IN SAID SOLUTION RANGING FROM AN EFFECTIVE NORMALITY UP TO 0.2 N.